Soil micro organisms and plant nutrition (or) I'll feed you and you feed me

GRDC Junior Research Fellow Megan Ryan discusses her work on soil micro-organism with David Dumaresq at the Australian National University.

Could farmers save the cost of soluble phosphate fertiliser if they encouraged some supportive fungi that are normally found attached to plant roots?

That's the intriguing possibility from a research project at the Australian National University, undertaken by GRDC Junior Research Fellow Megan Ryan.

Ms Ryan, working with agroecologist David Dumaresq, compared the VA mycorrhizal fungi populations found in a conventional system which uses soluble, chemical phosphate fertiliser with those in an 'organic' approach which uses a relatively insoluble mineral rock fertiliser.

(Mr Dumaresq, a farmer turned full-time researcher, is our guest columnist this issue, see pl5.)

The hypothesis was that chemical fertilisers de-activate the relationship between the plant and the fungus. A big part of that relationship is that mycorrhiza may play a role in making available to the plant relatively insoluble phosphorus found in soft rocks in the and," said Ms Ryan.

Ms Ryan's research involved two dryland mixed winter cropping and livestock operations on red-brown earth, with wheat as the major crop.

She found that on the conventional farm, the addition of soluble phosphorus allowed the plants easily to absorb sufficient phosphorus from close to their root systems.

"This left no role for the VA mycorrhizal fungi and they were inhibited from colonising the plant roots," she said.

On the organic farm application of the relatively insoluble reactive rock phosphate corresponded with a much higher level of VA mycorrhizal fungi throughout the cropping season.

Glasshouse and field trials confirmed the link between adding soluble phosphate fertilisers and low levels of VA fungi.

'Modern management techniques often replace or reduce the roles of soil micro-organisms," said Ms Ryan. She said research has been focused on developing new VA mycorrhizal strains and techniques for inoculating them into farm soils. But, she said large-scale methods of direct inoculation have not reached a practical stage.

She believes the alternative of encouraging natural populations is worth looking at more thoroughly. By doing so, farmers could save on fertiliser costs while improving their soils overall.

The benefits of VA mycorrhizal fungi

Vesicular-arbuscular (VA) mycorrhizal fungi live in and around the roots of a host plant. The plant and the fungus enjoy a mutually beneficial feeding relationship. In return for carbohydrates which the plant produces during photosynthesis, the fungus helps the plant obtain nutrients from the soil. The fungus is able to explore for nutrients more widely than the roots of the host plant. Researchers believe phosphorus is a major part of this mutual nutrient takeup.

The fungi are believed to provide water and other mineral nutrients such as copper and zinc to the plant and may inhibit other, disease-causing fungi from invading the plant.

In addition, the 'roots' of the Mycorrhizal fungi contribute to good soil structure through stabilising soil aggregates and are a food source for other soil organisms.

In sum, high levels of VA mycorrhizal fungi appear likely to be good allies for the long-term sustainable functioning of the farm.